Physical Properties Measurement And NH₃/H₂O Bubble Absorption Enhancement Of Binary Nanofluids

Nanofluids,which can enhance the thermal conductivity and mass diffusivity of the base fluid,is a novel approche to improve the heat and mass transfer in the NH₃/H₂O bubble absorption process,.The previous investigations are limited because of many encountered problems,such as the preparation of the specific nanofluids for the absorption process,the transportation properties of the binary nanofluids,the factors and the mechanism affecting the absorption enhancement.This dissertation mainly focuses on the following aspects for the NH₃/H₂O bubble absorption process using binary nanofluids.1)The carbon nanotubes(CNTs) are used as the nanoparticles,and the CNTs-NH₃ binary nanofluid is prepared without any surfactants addition.The physical properties,such as thermal conductivity,surface tension and kinematic viscosity of binary nanofluids,are measured.The effects of the mass fraction of carbon nanotubes,the concentration of ammonia and the temperature are systematically studied with a series of experiments.The results show that the CNTs-NH₃ binary nanofluids have remarkably higher effective thermal conductivities than that of the aqueous ammonia,and the thermal conductivity ratio between the binary nanofluid and the base fluid increases with the mass fraction of CNTs and the temperature increasing. However,the thermal conductivity ratios of CNTs-NH₃ nanofluids are lower than that of the CNTs-H₂O ones due to the influence of ammonia ions on the dispersion of CNTs in the base fluid.The addition of CNTs has only a slight effect on the surface tension and kinetic viscosity of the CNTs-NH₃ binary nanofluid,and the variation trend of these properties with the concentration of ammonia and the temperature is same with that of the aqueous ammonia.2) The effects of the nanoparticles on the mass diffusion are investigated experimentally by visualizing the diffusion process of a fluorescein in the SiO₂-H₂O nanofluid and the CNTs-H₂O nanofluid.The results show that the diffusion rates of the fluorescein in the two kinds of nanofluids are higher than those in water,and an optimum mass fraction of nanoparticles exists for the enhancement of the diffusion rate in the nanofluids.3) The experiments are conducted for the enhanced performance of the binary nanofluid for the NH₃/H₂O bubble absorption process.The effects of the mass fraction of CNTs,the initial concentration of ammonia and the flow rate of ammonia vapor on NH₃/H₂O bubble absorption characteristics are studied.The results show that the effective absorption ratio doesn't increase linearly with the mass fraction of nanoparticles,but increases firstly,and then decrease.Namely, there is an optimum mass fraction of nanoparticles for the enhancement of the bubble absorption.The effective absorption ratio increases with the initial concentration of ammonia increasing.The absorption potential of aqueous ammonia generally decreases with increasing the ammonia concentration,i.e.the lower the absorption potential is,the more significant the enhancement.The flow rate of ammonia vapor has little effect on the enhancement of bubble absorption in the binary nanofluid within the experimental range.Based on the previous study about the thermal physical properties of the binary nanofluid and the relevant reports in literature,the mechanism of the binary nanofluid for enhancing bubble absorption process is analyzed with respect to the effects of the nanoparticles on the heat transfer process,mass transfer process and the forming process of the bubbles in the NH₃/H₂O bubble absorption process.The presence of nanoparticles can improve the thermal conductivity and mass diffusivity of aqueous ammonia,stimulating the grazing effect,and increase the gas holdup of ammonia vapor in aqueous ammonia.And hence it can enhance the heat and mass transfer in the bubble absorption.4) The forming processes of the bubbles in two transparent nanofluids of SiO₂-H₂O and SiO₂-C₂H₅OH are observed by the high-speed CCD camera,and the changes of the bubble detaching size in the nanofluids are analyzed according to the dynamics of the bubble.The results show the presence of the SiO₂ nanoparticles decreases the radius of the bubble and increases the detaching frequency of the bubbles in the two nanofluids.Moreover,the radius of the bubble decreases with the mass fraction of the nanoparticles increasing.The smaller bubbles are contributed to the higher gas holdup at the same flow rate of the gas.Therefore,it can be proved that the nanoparticles affect assuredly the forming process of the bubbles and are one of the most important factors for the increase of the gas holdup in the nanofluid and for enhancing bubble absorption process.